1. Field of the Invention
This invention relates to a process for reducing the size of halftone dot images obtained by imagewise exposing and developing a metal image-forming material comprising a support, a thin metallic layer composed mainly of aluminum on the support, and a photosensitive resin layer on the thin metallic layer.
2. Description of the Prior Art
Japanese patent application (OPI) No. 139,720/75 corresponding to U.S. patent application, Ser. No. 571,817, filed Apr. 25, 1975, now U.S. Pat. No. 4008084, discloses a metal image-forming material comprising a support, a thin metallic layer composed mainly of aluminum on the support and a photosensitive resin layer on the thin metallic layer. When this metal image-forming material is exposed through a image using active radiation and then contacted with or immersed in an alkaline solution the resin layer is dissolved or swollen at the exposed portion or unexposed portion depending on the property of the photosensitive resin layer, and the alkaline solution permeates to the metallic layer to dissolve it and thus to form the corresponding image. The image so obtained has high contrast, and superior resolving power, and is especially suitable for preparing a halftone original for printing plates.
In an ordinary silver halide photographic material for printing (a so-called lithographic material), an operation is performed to partly reduce the size of the dots forming the halftone images in order to obtain better prints. This reducing operation is usually effected by coating a reducer solution containing a compound capable of dissolving the image (silver) on a part of the image after development. This halftone dot size reduction, for example, comprises the conversion of the silver image to a silver salt and removal of the silver salt by dissolution. It is inevitable as a result for the peripheral edges of the halftone dots to be etched away and their area decreased in size, and at the same time, the density of the halftone dots as a whole to decrease.
Since, however, the halftone dot size reduction of the above-described metal image-forming material is based on a so-called "side-etching method" by which the reducer solution permeates between the support and the photosensitive resin and contacts the metal layer at sites where such does not contact the photosensitive resin or the support thereby to etch the metal layer, this method offers the great advantage that only the sizes of the area of the halftone dots is changed without decreasing the optical density of the halftone dots as a whole, which advantage cannot be obtained by a dot reduction of conventional lithographic materials.
Because the space between the support and the photosensitive resin layer is very narrow in the above-described metal image-forming material, the penetration of the reducer solution is non-uniform causing an unevenness in the etching of the metal image to occur, and the shape of the halftone dots sometimes is deteriorated. This degradation of the shape of the halftone dots, that is, the reduced similarity of the shapes of the halftone dots before and after the halftone dot size reduction, causes the serious defect to occur that prints faithful to the original cannot be obtained.